TechDraw: Link related changes

* Support link and group objects

* Support view sync by implementing view provider API getMDIView()

* Use handleChangedPropertyType() for object migration instead of
  reimplementing Restore() because of a lots of changes in
  PropertyContainer::Restore().

* Various other small fixes.

src/Mod/TechDraw/App/DrawView.h

@@ -67,7 +67,8 @@ public:
     virtual void onDocumentRestored() override;
     virtual short mustExecute() const override;
     //@}
-    void Restore(Base::XMLReader &reader) override;
+    virtual void handleChangedPropertyType(
+        Base::XMLReader &reader, const char * TypeName, App::Property * prop) override;
 
     bool isInClip();
     DrawViewClip* getClipGroup(void);